Project Summary A major deficiency in understanding the role of B cell in lupus and in devising rational therapies targeting B cells is the gap in knowledge about the generation and fate of autoantibody-producing B cells. Previous studies have established that endosomal TLR signaling in B cells is central to the production of autoantibodies in lupus. This project seeks to apply this finding toward developing a new animal model of lupus that will make it possible to follow the developing endosomal TLR-dependent and autoantibody-producing B cells and their differentiation and progression over a long period. To accomplish this, we propose to develop a model in which B cells from lupus mice are transferred into lupus mice deficient in endosomal TLR signaling because of the 3d mutation. 3d is a function-ablating mutation in UNC93B1, a transport protein essential for endosomal TLRs. Initial results involving the transfer of B6-lpr IgHa CD45.1 B cells into B6-lpr 3d IgHb CD45.2 mice demonstrated a marked production of IgM and IgG2a from donor B cells as well as persistence of a small cohort of transferred B cells that over several weeks acquired phenotypic characteristics of memory B cells. For this project, two aims are proposed. Aim 1 will develop this model for studying eTLR-dependent B cell activation and autoAb production, and Aim 2 will define factors affecting eTLR-dependent B cell responses. Successful development of this system will make it possible to examine the development and long-term fate of autoantibody-producing B cells.